Secretary of State John Kerry greets U.S. marines as he arrives at the U.S. embassy in the International Zone on June 23 in Baghdad. / Brendan Smialowski, AFP/Getty Images

by Jim Michaels, USA TODAY

by Jim Michaels, USA TODAY

WASHINGTON â?? The first contingent of U.S. military advisers arrived in Iraq Tuesday to assess conditions and begin assisting Iraqi security forces after Islamist militants have seized several key cities.

The Pentagon said 90 American personnel arrived from outside the country, and 40 were already in Iraq as part of the U.S. Embassy's defense staff.

The advisers are part of up to 300 military personnel authorized by President Obama last week in response to the crisis in Iraq, where a Sunni extremist group, called the Islamic State of Iraq and the Levant (ISIL), has taken control of Mosul, Iraq's second-biggest city, and other towns in recent weeks.

"They're still a legitimate threat to Baghdad," Rear Adm. John Kirby, a Pentagon spokesman, said Tuesday about ISIL.

The troops arrived as Secretary of State John Kerry, on his second day in Iraq, met with the top Kurdish leader, Massoud Barzani, in Irbil, the capital of Iraq's northern Kurdish region.

Barzani told Kerry the successes of Sunni militants have already created "a new reality and a new Iraq," highlighting the challenges the U.S. faces in promoting unity in Iraq among its rival factions.

The United Nations said Tuesday that more than 1,000 people, most of them civilians, have been killed in Iraq this month - among the highest death tolls since the U.S. military withdrew from the country in December 2011.

Kerry said at the start of an hour-long meeting that the Kurdish security forces have been "really critical" in helping restrain the insurgents.

"This is a very critical time for Iraq, and the government formation challenge is the central challenge that we face," Kerry said. He said Iraqi leaders must "produce the broad-based, inclusive government that all the Iraqis I have talked to are demanding."

Kirby said the 40 military advisers reassigned from embassy jobs will form into two teams to begin assessing Iraq's military and the militants. The 90 from outside the country will begin setting up one of two joint information centers where U.S. and Iraqi personnel will share intelligence.

"We just don't have perfect visibility - that's the whole reason we're putting these teams on the ground," Kirby said.

The United States has also boosted surveillance flights over Iraq and is now flying between 30 and 35 missions a day, using both drones and manned aircraft, Kirby said.

The White House has not ruled out other military options, including airstrikes, Kirby noted.

A challenge for the United States is that any effort to help Iraq's security forces could be viewed in the region as picking sides in a civil war.

The violence has taken on sectarian overtones. Sunnis say they have been marginalized by the Shiite-dominated government of Prime Minister Nouri al-Maliki.

Kerry's visit to Baghdad on Monday and Irbil on Tuesday is to urge political leaders to put aside sectarian differences and form a more inclusive government.